1. Field of the Invention
The present invention relates to signal transmission systems using code division multiple access (CDMA) modulation techniques, and more particularly, to an initial synchronization acquisition method in a process for accomplishing synchronization of diffusion codes in a receiver of a CDMA transmission system, and an apparatus therefor.
2. Description of the Related Art
CDMA is a communications method of making a transmission channel by modulating data bits to be transmitted into diffusion codes, that is, a band diffusion digital communications method of making several transmission channels using several diffusion codes at one time. A diffusion code operates at a chip rate that is significantly higher than a data bit rate, to band-diffuse data to be transmitted. Signals of several channels can be multiplexed using the auto correlation characteristics and the cross correlation characteristics of the diffusion codes, since pseudo noise (PN) codes, which are orthogonal or quasi-orthogonal to each other, are used as the diffusion code.
The main function of PN code synchronization in CDMA is to reverse-diffuse a received signal to demodulate the same. The received signal is essentially made up of two types of digital signals. One type of signal is an information signal such as a coded audio signal, and the other type of signal, which is a PN code generated by a PN code generator, has a bit rate that is significantly higher than the information signal.
A receiver reverse-diffuses a received signal using PN codes which are generated by a local PN generator, and synchronizes the PN code with a PN code component included in the received signal. The PN code component is removed from the received signal, and then the received signal from which the PN code component has been removed is integrated for a symbol period. In this way, the original information signal can ideally be obtained.
Code synchronization usually includes the following two steps: (1) the first code synchronization acquisition step of arranging the phase of a PN code included in a received signal with the phase of a locally-generated PN code within one code chip period; and (2) the second code phase tracing step of arranging two PN code phases at accurate positions using a phase locked loop (PLL).
The present invention is directed to the first step of code sync acquisition. The code sync acquisition step is important in CDMA systems.
Techniques for code sync acquisition using several types of search and determination methods have been proposed up to now because of the importance of code sync acquisition. The proposed code sync acquisition techniques can be largely classified into the following types of techniques.
The first type is a parallel search method, in which a received signal is simultaneously correlated with all possible code phases of a locally-generated PN code in parallel, and it is determined in parallel whether the received signal is synchronized with each of the code phases. This method can reduce a code sync acquisition time, but complicates hardware.
The second type is a serial search method, in which a determination as to whether a received signal is synchronized with a locally-generated PN code is made by comparing a correlation value obtained by correlating the received signal with the locally-generated PN code, with a specific threshold. If it is determined that the received signal is synchronized with the locally-generated PN code, a code phase tracing process starts, and if it is determined otherwise, the above-described determination process is performed again after changing the phase of the internally-generated PN code. As described above, the search is performed with respect to all PN code phases that can be generated. This type of method can be considered simple compared to the parallel search method in terms of hardware, but it increases the sync acquisition time.
The time required for completing the code synchronization acquisition process, and the accuracy of synchronization, are important factors restricting the performance of CDMA receivers. Generally, the code sync acquisition process is very difficult among processes performed in CDMA systems because of a poor channel environment such as a low signal-to-noise ratio (SNR), Doppler effects and a fading environment. Among these distortion factors, a channel change due to the Doppler effect is one concern addressed by the present invention.
In a wireless mobile channel environment, the Doppler effect, which is generated when a receiver or peripheral objects move relative to a transmittar, causes a change in the channel power and channel phase with respect to the lapse of time. The variations are proportional to the speed of the moving body. When the amplitude of a received signal varies with the change in the channel power, a correlation energy value between a received signal and a local PN code also varies.
Accordingly, when the serial search method is used as in the prior art, code phases cannot be searched in the same environment since the magnitude of a received signal varies every time each local PN code phase is searched. If a local PN code phase being currently searched is an accurate code phase, the correlation energy value thereof may be significantly greater than that of the previous local PN code phase (theoretically, greater by a multiple of the processing gain of the receiver.
However, if the current channel power is significantly smaller than the previous channel power due to a change in the channel power, even though the code phase being currently searched is an accurate code phase, the correlation energy value thereof is also very small, since the amplitude of a received signal is too small. Considering the worst case, the correlation energy value of the current code phase may be equal to or, even, smaller than that of the previous inaccurate code phase.
In these circumstances, proper acquisition of code synchronization is very difficult even when using an adaptive threshold, not to mention the case of determining whether code synchronization has been achieved using a fixed threshold. A code synchronization acquisition technique using an adaptive threshold adaptively obtains a determination threshold whenever continuously calculating a change in channel power. However, this code sync acquisition technique cannot calculate a change in channel power in real time, so that it is difficult to apply adaptively-obtained thresholds at appropriate times and make a determination. Also, under circumstances of having a significantly low SNR such as in a wireless mobile channel environment, it is difficult to properly obtain a change in the channel power, so that even an adaptively-obtained determination threshold cannot be considered a correct value.
An example of an existing serial search method using a fixed threshold is disclosed in U.S. Pat. No. 5,644,591, issued Jul. 1, 1997, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATIONS SYSTEMxe2x80x9d, assigned to Qualcomm Incorporated. An example of the existing serial search method using an adaptive threshold is disclosed in U.S. Pat. No. 5,642,377, issued Jun. 24, 1997, entitled xe2x80x9cSERIAL SEARCH ACQUISITION SYSTEM WITH ADAPTIVE THRESHOLD AND OPTIMAL DECISION FOR SPREAD SPECTRUM SYSTEMSxe2x80x9d, assigned to Nokia Mobile Phones, Ltd.
Current parallel search methods also cause problems of existing serial search method. That is, the existing parallel search method has the same determination technique as the serial search method, except that the correlation energies of several code phases are simultaneously obtained. Thus, in current parallel search methods, it is difficult to solve problems due to changes in channel power.
To solve the above problems, a feature of the present invention includes providing a code synchronization acquisition method and device in a code division multiple access (CDMA) transmission system, by which stable code synchronization acquisition is provided, and the total time for initial code synchronization acquisition can be reduced, by drastically reducing the probabilities of false alarm and mis-detection which are caused due to a change in the power of a CDMA received signal, without being affected by noise included in the CDMA received signal, in the initial code synchronization acquisition step in a diffusion code synchronization process, in a receiver for receiving a CDMA signal using a wireless mobile channel as a transmission medium.
The invention comprises a method of acquiring initial diffusion code synchronization after receiving in a receiver CDMA-modulated signal; down-converting the CDMA signal into an analog signal; sampling the analog signal to produce a complex digital signal; performing N parallel complex correlations to obtain a correlation between the sampled complex digital signal and N parallel complex diffusion codes generated by the receiver; accumulating by components K continuously-generated parallel complex correlation results using in parallel the N parallel complex correlations; obtaining in parallel energy values of the components of the accumulating step; determining the ratio of a maximum energy value to a mean energy value using the energy values of the components; comparing the ratio with a predetermined determination threshold, and, if the ratio is greater than or equal to the determination threshold, concluding diffusion code synchronization by determining that a code phase corresponding to the maximum energy value is a correct code phase, and otherwise, determining that the code phase corresponding to the maximum energy value is an incorrect code phase.
The present invention also comprises a device for acquiring initial diffusion code synchronization after receiving in a receiver a CDMA-modulated signal, down-converting the CDMA-modulated signal into an analog signal, and sampling the analog signal to produce a complex digital signal wherein the device comprises a parallel complex correlator for generating N parallel complex correlation results indicative to the correlation between the sampled complex digital signal and N parallel complex diffusion codes generated by the receiver; a parallel complex accumulator for accumulating in parallel K continuously-generated parallel complex correlation results; a parallel energy detector for obtaining in parallel the energy values of K accumulated parallel complex correlation results; and an adaptation ratio determiner for obtaining the ratio of a maximum energy value to a mean energy value using the energy values of the K accumulated parallel complex correlation results, comparing the ratio with a predetermined determination threshold, and, if the ratio is greater than or equal to the determination threshold, generating a search conclusion signal by determining that a code phase corresponding to the maximum energy value is a correct code phase.
The invention, though, is pointed out with particularity by the appended claims.